WASHINGTON -- The populations of 20 common American birds -- from the fence-sitting meadowlark to the whippoorwill with its haunting call -- are half what they were 40 years ago, according to an analysis released Thursday.

Suburban sprawl, climate change and other invasive species are largely to blame, said the study's author Greg Butcher of the National Audubon Society.

"Most of these we don't expect will go extinct," he said. "We think they reflect other things that are happening in the environment that we should be worried about."

Last month a different group of researchers reported that seven species had dramatically declined because of West Nile virus. The species harmed by West Nile are different from those listed in the new study -- except for the little chickadee, hard-hit on both lists.

Many of the species listed as declining in the new study depend on open grassy habitats that are disappearing, said Butcher, Audubon's bird conservation director.

Some of the birds, such as the evening grosbeak, used to be so plentiful that people would complain about how they crowded bird-feeders and finished off 50-pound sacks of sunflower seeds in just a couple days. But the colorful and gregarious grosbeak's numbers have plummeted 78 percent in the past 40 years.

"It was an amazing phenomena all through the '70s that's just disappeared. It's just a really dramatic thing because it was in people's back yards and (now) it's not in people's back yards," said Butcher.

For the study, researchers looked at bird populations of more than half a million which covered a wide range. They compared databases for 550 species from two different bird surveys -- the Audubon's own Christmas bird count and the U.S. Geological Survey's breeding bird survey in June. The numbers of 20 different birds were at least half what they were in 1967.

The northern bobwhite and its familiar wake-up whistle once seemed to be everywhere in the East. Last Christmas, volunteer bird counters could find only three of them and only 18 Eastern meadowlarks in Massachusetts.

The bobwhite had the biggest drop among common birds. In 1967, there were 31 million of this distinctive plump bird. Now they number closer to 5.5 million.

"Things we all think of as familiar backyard birds ... they appear in books and children's stories and suddenly some of them are way less familiar than they should be," said John Fitzpatrick, director of the Cornell ornithology lab, who was not part of the study.

Audubon Board Chairman Carol Browner, former head of the U.S. Environmental Protection Agency, called the declines "a warning signal."

"We are concerned. Is it an emergency? No, but concerns can quickly become an emergency," she said.

While these common birds are in decline, others are taking their place or even elbowing them aside. The wild turkey, once in deep trouble, is growing at a rate of 14 percent a year. The double-crested cormorant, pushed nearly to extinction by DDT, is growing at a rate of 8 percent a year and populations of the pesky Canada goose increase by 7 percent yearly.

Many of the birds that are disappearing are specialists, while the thriving ones are generalists that do well in urban sprawl and all kinds of environments, Butcher said. In a way it's the Wal-Mart-ization of America's skies, he said.

"The robins, the Carolina wrens, the blue jays, the crows, those kinds of birds, are doing just fine, thank you," Butcher said. "They really get along in suburban habitats, most of them even like city parks, so they are not as susceptible to the human changes in environment."

But nothing matches the take-over ability of one invading bird.

"Right now the Eurasian collared-dove is conquering America," Butcher said. A dove-like bird that first entered Florida in the 1980s, it now is the most prevalent bird in the Sunshine State and is in more than 30 states.

"Soon you'll be seeing Eurasian collared-doves in any city in the world," he said.